Deprecating arithmetic on std::atomic<void*>

[Florian Weimer]

On 04/19/2017 07:07 PM, Jonathan Wakely wrote:
> I know it's a bit late, but I'd like to propose deprecating the
> libstdc++ extension that allows arithmetic on std::atomic<void*>.
> Currently we make it behave like arithmetic on void*, which is also a
> GNU extension (https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html).
> We also allow arithmetic on types such as std::atomic<void(*)()> which
> is probably not useful (PR 69769).

Why is it acceptable to have the extension for built-in types, but not 
for library types wrapping them?  Why be inconsistent about this?

Thanks,
Florian